Improved amylases can ameliorate problems inherent in certain processes, such as baking. Crystallisation of amylopectin takes place in starch granules days after baking, which leads to increased firmness of bread and causes bread staling. When bread stales, bread loses crumb softness and crumb moisture. As a result, crumbs become less elastic, and bread develops a leathery crust.
Enzymatic hydrolysis (by amylases, for example) of amylopectin side chains can reduce crystallization and increase anti-staling. Crystallization depends upon the length of amylopectin side chains: the longer the side chains, the greater the crystallization. Most starch granules are composed of a mixture of two polymers: amylopectin and amylose, of which about 75% is amylopectin. Amylopectin is a very large, branched molecule consisting of chains of α-D-glucopyranosyl units joined by (1-4) linkages, where the chains are attached by α-D-(1-6) linkages to form branches. Amylose is a linear chain of (1-4) linked α-D-glucopyranosyl units having few α-D-(1-6) branches.
Baking of farinaceous bread products such as white bread, bread made from bolted rye flour and wheat flour and rolls is accomplished by baking the bread dough at oven temperatures in the range of from 180 to 250° C. for about 15 to 60 minutes. During the baking process a steep temperature gradient (200→120° C.) prevails over the outer dough layers where the crust of the baked product is developed. However, due to steam, the temperature in the crumb is only about 100° C. at the end of the baking process. Above temperatures of about 85° C., enzyme inactivation can take place and the enzyme will have no anti-staling properties. Only thermostable amylases, thus, are able to modify starch efficiently during baking.
Endoamylase activity can negatively affect the quality of the final bread product by producing a sticky or gummy crumb due to the accumulation of branched dextrins. Exoamylase activity is preferred, because it accomplishes the desired modification of starch that leads to retardation of staling, with fewer of the negative effects associated with endoamylase activity. Reduction of endoamylase activity can lead to greater exospecifity, which can reduce branched dextrins and produce a higher quality bread.
The present invention is drawn to nucleic acids encoding polypeptides which have altered exo-specificity and/or thermostability, the encoded polypeptides and their uses.